Liquid crystalline light modulating device and material

ABSTRACT

A new liquid crystalline light modulating cell and material are characterized by liquid crystalline light modulating material of liquid crystal and polymer, the liquid crystal being a chiral nematic liquid crystal having positive dielectric anisotropy and including chiral material in an amount effective to form focal conic and twisted planar textures, the polymer being distributed in phase separated domains in the liquid crystal cell in an amount that stabilizes the focal conic and twisted planar textures in the absence of a field and permits the liquid crystal to change textures upon the application of a field. In one embodiment, the material is light scattering in a field-OFF condition and optically clear in a field-ON condition, while in another embodiment, the material is optically clear in a field-OFF condition and light scattering in a field-ON condition. In still another embodiment, the material exhibits stability at zero field in a colored, light reflecting state, a light scattering state and multiple stable reflecting state therebetween, as well as being optically clear in the presence of a field.

This application was made in part with Government support undercooperative agreement number DMR 89-20147 awarded by the NationalScience Foundation. The Government has certain rights in this invention.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/425,897,filed Apr. 21, 1995, which now abandoned which is a division ofapplication Ser. No. 07/969,093 filed Oct. 30, 1992 now U.S. Pat. No.5,437,811 is a continuation in part of U.S. Ser. No. 07/694,840 filedMay 2, 1991, now abandoned incorporated herein by reference, and U.S.Ser. No. 07/885,154, filed May 18, 1992, now U.S. Pat. No. 5,284,067,incorporated herein by reference.

BACKGROUND OF THE INVENTION TECHNICAL FIELD

The present invention relates generally to liquid crystalline lightmodulating devices, and more specifically to new phase-separatedpolymeric-liquid crystalline display cells and materials which exhibitdifferent optical states under different electrical field conditions andare characterized by a unique combination of properties, includingoptical multistability and haze-free light transmission at all viewingangles in either a field-ON or field-OFF mode.

DESCRIPTION OF THE RELATED ART

Electrically switchable liquid crystal-polymer films intended for use invarious electro-optical devices have been prepared by mechanicalentrapment procedures. One such technique involves imbibing liquidcrystal into micropores of a plastic or glass sheet. Another techniqueinvolves evaporation of water from an aqueous emulsion of nematic liquidcrystal in a solution of water-soluble polymer such as polyvinyl alcoholor in a latex emulsion.

A different procedure offering significant advantages over mechanicalentrapment techniques and the emulsification procedure involves phaseseparation of nematic liquid crystal from a homogeneous solution with asuitable synthetic resin to form a liquid crystal phase dispersed with apolymer phase. The resulting materials are referred to as polymerdispersed liquid crystal (PDLC) films. Some of the advantages of PDLCfilms are discussed in U.S. Pat. Nos. 4,671,618; 4,673,255; 4,685,771;and 4,788,900; the disclosures of which are incorporated by reference.PDLC films have been shown to be useful in many applications rangingfrom large area displays and switchable coatings for windows toprojection displays and high-definition television.

The methods of phase separation can be carried out by polymerizationinitiated by addition of a curing agent, by ultraviolet light or bycooling into the region of immiscibility. Another method is evaporatinga solvent from a matrix-producing composition of a solution of polymerand liquid crystal in the solvent.

In windows or displays as described above in which the ordinary index ofrefraction of the liquid crystal is matched to the refractive index ofthe polymer, the device appears most transparent (field-ON state) whenviewed along the direction of the field which is usually normal to theviewing surface. Transparency decreases giving rise to increasing "haze"at increasing oblique viewing angles until an essentially opaqueappearance is detected at an oblique enough angle. This condition ofhaze results from the fact that the farther the viewing angle is fromthe orthogonal, the greater is the perceived mismatch between theeffective index of refraction of the liquid crystal and the refractiveindex of the matrix.

A further development of PDLC films disclosed in U.S. patent applicationSer. No. 07/324,051, now U.S. Pat. No. 4,994,204 issued Feb. 19, 1991,involves the use of a birefringent polymer, e.g., a liquid crystalpolymer. The PDLC film prepared with the birefringent polymer has thecharacteristic of displaying haze-free transparency for all directionsof incident light. This is accomplished by matching the ordinary andextraordinary indices of refraction of the polymer to the ordinary andextraordinary indicts of refraction of the liquid crystal.

PDLC films made with birefringent polymer can operate in the normalmanner so that they are clear in a field-ON state and light scatteringin a field-OFF state. Alternatively, the films can be made to operate ina reverse or "fail-safe" mode such that the material is clear in theabsence of a field and is light scattering in the field-ON state.

DISCLOSURE OF THE INVENTION

The invention is an electrically switchable material which exhibits aunique combination of properties that afford significant advantages overpreceding technology. For example, the new material can operate eitherin the mode of being light scattering in a field-OFF condition and clearin a field-ON condition or in the reverse mode of being clear in thefield-OFF condition and light scattering in the field-ON condition. Inboth instances, the material exhibits minimal haze at all viewing angleswhen in the clear state.

Another important feature of the invention is that the material can beprepared so that it exhibits multiple optically different states, all ofwhich are stable in the absence of an applied field. When incorporatedin a display device, the material can be driven from one state toanother by an electric field. Depending upon the magnitude and shape ofthe electric field pulse, the optical state of the material can bechanged to a new stable state which reflects any desired intensity ofcolored light along a continuum of such states, thus providing a stable"grey scale." A low electric field pulse results in a light scatteringstate which is white in appearance. The application of a sufficientlyhigh electric field pulse, i.e., an electric field high enough tohomeotropically align the liquid crystal directors, drives the materialto a light reflecting state that can be any desired color. The lightscattering and light reflecting states remain stable at zero field. If asufficiently high electric field is maintained, the material istransparent until the field is removed. When the field is turned offquickly, the material reforms to the light reflecting state and, whenthe field is turned off slowly, the material reforms to the lightscattering state. Electric field pulses of various magnitudes below thatnecessary to drive the material from the stable reflecting state to thestable scattering state will drive the material to intermediate statesthat are themselves stable. These multiple stable states indefinitelyreflect colored light of an intensity between that reflected by thereflecting and scattering states. Thus, depending upon the magnitude ofthe electric field pulse the material exhibits stable grey scalereflectivity. Application of mechanical stress to the material can alsobe used to drive the material from the light scattering to the lightreflecting state.

A major advantage of the multistable material is that it does notrequire an active matrix to make a high-definition flat panel screen.The screen can be prepared without active elements at each pixel siteand a multiplexing scheme used to address the display. This greatlysimplifies production, increases yield and reduces the cost of thedisplay.

Multiplexed flat panel liquid crystal displays are not new and have beendeveloped primarily with super twisted nematic materials forapplications such as lap-top computer screens where speed, contrast orcolor is not an important issue. Ferroelectric liquid crystals, whichexhibit a surface stabilized bistable state, also can be multiplexed.These displays have been difficult to commercialize because the surfacestabilization is not maintained under severe operating conditions. Thematerial of the present invention provides several advantages in thatthe light scattering and light reflecting states are materiallystabilized without requiring delicate surface conditions of thesubstrate. Display devices made with the material of the invention donot require polarizers which limit the brightness of the displays.Furthermore, color is introduced by the material itself without the needof color filters which also can reduce brightness.

The advantageous properties described above are achieved in theinvention by providing a light modulating cell comprising a liquidcrystalline light modulating material of liquid crystal and polymer, theliquid crystal being a chiral nematic liquid crystal having positivedielectric anisotropy and including chiral material in an amounteffective to form focal conic and twisted planar textures, the polymerbeing distributed in phase separated domains in the cell in an amountthat stabilizes the focal conic and twisted planar textures in theabsence of a field and permits the liquid crystal to change texturesupon the application of a field.

The addressing means can be of any type known in the art, such as anactive matrix, a multiplexing circuit, electrodes, etc. The liquidcrystal molecules in the vicinity of the polymer domains are anchored bythe polymer. As a result, the new material can be made to exhibitdifferent optical states, i.e., light transmitting, light scattering,light reflecting and stable grey scale in between these states, underdifferent field conditions.

The material used to form the polymer networks is soluble with thechiral nematic liquid crystal and phase separates upon polymerization toform phase separated polymer domains. Suitable polymer materials may beselected from U.V. curable, thermoplastic and thermosetting polymers,including polymers formed from monomers having at least twopolymerizable double bonds so as to be cross-linkable,polymethylmethacrylates, bisacrylates, hydroxyfunctionalizedpolymethacrylates and epoxy systems to name a few. The amount of polymerto be used depends upon the polymer. Useful results have been obtainedwith polymer contents ranging from about 1.5 to about 40% depending uponthe polymer.

The chiral nematic liquid crystal is a mixture of nematic liquid crystalhaving positive dielectric anisotropy and chiral material in an amountsufficient to produce a desired pitch length. Suitable nematic liquidcrystals and chiral materials are commercially available and would beknown to those of ordinary skill in the art in view of this disclosure.The amount of nematic liquid crystal and chiral material will varydepending upon the particular liquid crystal and chiral material used,as well as the desired mode of operation. For normal and reverse modecells, useful results can be obtained using from 0.5 to about 17% byweight chiral material based on the combined weight of nematic liquidcrystal and chiral material and depending upon the chiral material used.A preferred range of chiral material is from about 1 to about 16%. Formultistable cells, useful results have been obtained using from about 18to about 66% by weight chiral material based on the combined weight ofchiral material and nematic liquid crystal.

The wavelength of the light that is reflected by the material is givenby the relation λ=np, where n is the average refractive index and p isthe pitch length. Wavelengths above 800 nm are in the infrared and thosebelow 380 nm are in the ultra violet. In cells which operate in eitherthe normal mode of being light scattering in a field-OFF condition andlight transmitting in a field-ON condition or the reverse mode of beinglight transmitting in the field-OFF condition and light scattering in afield-ON condition, the chiral nematic liquid crystal has a pitch lengtheffective to reflect light outside the visible spectrum, preferably inthe infrared spectrum. A preferred pitch length for normal mode andreverse mode cells ranges from about 1.0 to about 4.0 microns. Liquidcrystalline light modulating materials that operate in the normal andreverse modes have been prepared with chiral nematic liquid crystalcontaining from about 1 to about 16% by weight, chiral material based onthe combined weight of nematic liquid crystal and chiral material. Itwill be understood that, in both instances, the weight amounts can varydepending upon the particular liquid crystal, chiral material andpolymer used.

In carrying out the invention, the solution of liquid crystal andpolymer (or polymer precursor) is introduced into a cell. Polymerizationis initiated in any suitable manner, as by UV radiation, thermally etc.,depending upon the polymer used. Under polymerization conditions, thepolymer phase separates from the chiral nematic liquid crystal and formsphase separated polymer domains of polymer molecules.

While not necessary to the invention, in some instances it is preferableto treat the cell walls to provide for surface alignment of the liquidcrystal molecules parallel to the cell walls, e.g., by providing thecell walls with rubbed polyimide layers or treating them with detergentor chemicals. This has the effect of improving transmission and responsetime in some reverse mode cells in the field-OFF condition.

In the case of normal mode cells, polymerization takes place in thepresence of an electric field that aligns the liquid crystal moleculesorthogonally to the cell walls. When polymerization has been completedand the electric field removed, the liquid crystal molecules in thevicinity of the polymer domains are anchored in a preferentialhomeotropic alignment. The surrounding chiral liquid crystal has a focalconic texture which results from competition between the forces in thecell, such as any surface effects of the cell walls, the electric fieldand the constraining effect of the polymer domains. In the field-OFFcondition, the polymer-liquid crystalline material is strongly lightscattering and the cell is opaque. In the field-ON condition, the focalconic texture reforms to homeotropic alignment so that the cell isoptically clear. There is negligible variation or fluctuation ofrefractive index throughout the liquid crystalline material because ofthe homeotropic alignment of the liquid crystal molecules and thetypically small amount of polymer in the composition. Therefore, thecell is haze-free at all viewing angles. However, it should be notedthat increasing the amount of polymer can have the effect of increasingthe amount of haze.

In the case of reverse mode cells, polymerization takes place in theabsence of a field. The liquid crystal molecules throughout the cellprefer a twisted planar structure. In the absence of a field, the cellis optically clear, since there is no reflecting or scattering in thevisible light region. In a field-ON condition, the liquid crystalmolecules have a focal conic texture in the presence of the field as aresult of competition of the various forces in the cell, such as anysurface effects, the electric field and the constraint of the polymerdomains. In this condition, the cell is light scattering. For liquidcrystal materials having suitably long pitch lengths, the material willreturn to the planar texture upon removal of the field.

The multistable color display cells are prepared by polymerizing andphase separating the liquid crystal-polymer solution either in zerofield or in a field effective to align the liquid crystal directors. Inboth instances, the polymer domains that are created in the materialserve to stabilize the light scattering state resulting from applicationof a low electric field pulse and the light reflecting state resultingfrom application of a high electric field pulse.

In the field-OFF condition with the liquid crystal molecules in atwisted planar texture parallel to the cell walls, the cell is in acolored light reflecting state. This state can be made to appear asgreen, red, blue, or any pre-selected color depending upon the pitchlength of the chiral nematic liquid crystal. When a low electric field,e.g. 6 volts per micron of thickness, is applied to the cell, it willswitch to a white, light scattering state. In this state, the liquidcrystal molecules surrounding the polymer domains have a focal conictexture as a result of the competition of any surface effects, theelectric field and the constraint of the polymer domains. The materialwill remain in the light scattering state when the low electric field isremoved. If a higher electric field, e.g. 12 volts per micron ofthickness, is applied to the cell, the material becomes optically clearuntil the voltage is removed. If the electric field is turned offquickly, the material switches to the uniform twisted planar structurewhich has the pre-selected color dictated by the pitch length. The lightreflecting state remains stable at zero field condition. If the field isturned off slowly, the material changes to its light scattering statewhich also remains stable at zero field condition. The effect of thepolymer domains is to stabilize both the planar and focal conic tenuresin the zero field condition. The magnitude of the field necessary todrive the material between various states will, of course, varydepending upon the nature and amount of the particular liquid crystaland polymer used, but could be easily determined by one of ordinaryskill in the art in view of the instant disclosure.

In the multistable color displays the chiral nematic liquid crystal hasa pitch length in a preferred range of from about 0.25 to 0.44 micronseffective to reflect circularly polarized colored light. Typical pitchlengths are 0.27 microns for blue color, 0.31 microns for green colorand 0.40 microns for red color. Multistable color display materials havebeen prepared containing from about 27 to about 66% (5% chiral materialbased on the combined weight of nematic liquid crystal and chiralmaterial. The ranges can vary, however, depending upon the chiralmaterial, liquid crystal and the polymer used.

In one embodiment the multistable display materials can be prepared tofunction as a bistable light shutter. By adjusting the pitch length ofthe chiral nematic liquid crystal to reflect light in the ultra violetrange, the material will appear clear when switched to the stable planartexture because the reflected light is outside the visible spectrum. Aswith the color reflecting cells, the material will scatter light whenswitched to the stable focal conic texture. Hence, the multistablematerial can be switched between a stable optically clear state, wherethe liquid crystal reflects light in the ultra violet range, and astable light scattering state. Pitch lengths effective to reflect lightin the ultra violet range will typically be from about 0.5 to about 1micron. Bistable light shutters that reflect light in the ultra violet,and hence appear clear in the planar texture, and scatter light in thefocal conic texture have been prepared containing about 18% by weightchiral material based on the combined weight of chiral material andnematic liquid crystal.

Surprisingly, the multistable color reflecting material exhibits astable grey scale, i.e., multiple optical states characterized byvarying degrees of intensity of reflection, all of which are stable inthe absence of an applied field. In between the reflecting andscattering states the material exhibits stable grey scale reflectance ofthe colored light depending upon the voltage of the electric fieldaddressing pulse. In each case, the electric field pulse is preferablyan AC pulse, and more preferably a square AC pulse, since a DC pulsewill tend to cause ionic conduction and limit the life of the cell.

Accordingly, the invention also features a method of addressing apolymer stabilized chiral nematic liquid crystal material capable ofbeing switched between a color reflecting state that reflects a maximumreference intensity, and a light scattering state exhibiting a minimumreference intensity. The method comprises applying voltage pulses ofvarying magnitude sufficient to achieve stable color reflectivitybetween said maximum and minimum, thereby producing stable grey scalereflectance from the material.

Preferably the method is characterized by subjecting the material to anAC pulse of sufficient duration and voltage to cause a proportion ofsaid chiral nematic material to exhibit a first optical state and theremaining proportion of the chiral nematic material to exhibit a secondoptical state that is different than the first state. In the preferredembodiment, the proportion of the material in the first optical stateexhibits the planar texture and the remainder of the material in thesecond optical state exhibits the focal conic texture, the intensity ofreflection being proportional to the amount of the material in theplanar reflecting texture.

Many additional features, advantages and a fuller understanding of theinvention will be had from the following detailed description ofpreferred embodiments and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic, cross-sectional illustration of a lightmodulating cell incorporating the polymer-liquid crystalline material ofthe invention.

FIG. 2 is a diagrammatic, fragmentary, enlarged cross-sectionalillustration of the new material when the liquid crystal ishomeotropically aligned to affect an optically clear state.

FIG. 3 is a diagrammatic, fragmentary, enlarged cross-sectionalillustration of the material in a light scattering state wherein theliquid crystal in proximity to the polymer domains is homeotropicallyaligned, while the surrounding liquid crystal has a focal conic texture.

FIG. 4 is a diagrammatic, fragmentary, enlarged cross-sectionalillustration of the material when the liquid crystal has a twistedplanar texture.

FIG. 5 is a diagrammatic, fragmentary, enlarged cross-sectionalillustration of the material wherein the liquid crystal in proximity tothe polymer domains has a twisted planar structure, while thesurrounding liquid crystal has a focal conic texture.

FIG. 6 is a plot of the dynamic response of a cell to AC pulses ofvarying voltages demonstrating grey scale reflection in the voltagerange of about 20 and 34 volts.

DESCRIPTION OF PREFERRED EMBODIMENTS

The diagrammatically illustrated cell in FIG. 1 comprises glass plates10, 11 which are sealed around their edges and separated by spacers 12.As shown, the glass plates 10, 11 are coated with indium-tin oxide orthe like to form transparent electrodes 13. The reference character 14represents an optional rubbed polyimide coating which can be applied tothe electrodes in order to affect homogeneous surface alignment of theliquid crystal directors.

The cell of FIG. 1 is filled with the polymer-liquid crystallinematerial of the invention. The liquid crystalline light modulatingmaterial is generally comprised of phase-separated polymer domains 15dispersed in surrounding chiral nematic liquid crystal 16 havingpositive dielectric anisotropy. An AC voltage source 17 is shownconnected to the electrodes 13 in order to switch the cell betweendifferent optical states.

It is to be understood that the form of the cell depicted in FIG. 1 hasbeen chosen only for the purpose of describing a particular embodimentand function of the liquid crystalline light modulating material of theinvention, and that the material can be addressed in various ways andincorporated in other types of cells. For example, instead of beingaddressed by externally activated electrodes, the new material can beaddressed by an active matrix, a multiplexing scheme or other type ofcircuitry, all of which will be evident to those working in the art.Similarly, the cells can be prepared without the optional alignmentlayers.

In accordance with the invention, the polymer domains 15 are defined bypolymer which is phase separated from a solution with the chiral nematicliquid crystal. The chiral nematic liquid crystal in proximity to thepolymer domains 15 is anchored by the polymer.

The polymer content in terms of weight based on the combined weight ofchiral nematic liquid crystal and polymer will vary depending upon thepolymer used, and is preferably present in an amount ranging from about1.5 to about 40% by weight based on the combined weight of polymer andliquid crystal. For example, cells have been prepared with a polymercontent ranging from about 1.5% to 10% using certain bisacrylates, fromabout 20 to 30% using certain hydroxy functionalized polymethacrylates,and about 40% when certain epoxies, thermoplastics and U.V. curedpolymers are used. It is to be understood, therefore, that the polymercontent is subject to some variation, in as much as what constitutes adesirable or undesirable appearance of the cell in its various opticalstates is a matter of subjective judgment.

In a preferred manner of preparing the cell shown in FIG. 1, the polymer(or its precursors, e.g. monomers) is dissolved with the chiral nematicliquid crystal together with any necessary photo-initiator, crosslinkingor curing agent. The solution is then introduced between the glassplates 10, 11, shown here having the optional rubbed polyimide coatings14. The solution is then polymerized in situ to induce concomitant phaseseparation of the polymer to form the polymer domains in the cell,conceptually illustrated by reference character 15. The polymerizationof the polymer-liquid crystal solution can take place either in thepresence of an electric field effective to homeotropically align theliquid crystal directors or in zero field. In the latter case, theliquid crystal molecules will prefer a twisted planar texture orientedparallel to the cell walls.

Normal Mode Cells

Normal mode cells which scatter light in a field-OFF condition and areoptically clear in a field-ON condition arc prepared using a chiralnematic liquid crystal effective to reflect light outside the visiblespectrum, preferably in the infrared spectrum. A preferred pitch lengthranges from about 1.0 to about 4.0 microns. Liquid crystalline lightmodulating materials having the desired pitch length may contain fromabout 1 to about 16% by weight chiral material based on the combinedweight of nematic liquid crystal and chiral material; although, theweight amounts can vary depending upon the particular liquid crystal,chiral material and polymer which are used.

Normal mode cells are prepared by polymerizing the polymer-liquidcrystal solution in the presence of an electric field. As shown in FIG.2, the electric field is effective to untwist the chiral nematic liquidcrystal molecules and homeotropically align the liquid crystal directors20. A single polymer domain 15 is conceptually illustrated in FIG. 2.

Each of the polymer domains 15 is believed to be a complex, oftencross-linked, three-dimensional network. When the electric field isturned off as illustrated in FIG. 3, the liquid crystal in the vicinityof the polymer tends to remain homeotropically aligned because of theanchoring affect of the polymer. The surrounding liquid crystalindicated by reference numeral 30 tends to reform to a focal conictexture, i.e., helically twisted molecules having randomly orientedhelical axes. The focal conic texture results from competition betweenthe various forces in the system such as any surface effects and theconstraining effect of the polymer domains on the liquid crystal. In thefield-OFF condition illustrated in FIG. 3, the polymer-liquidcrystalline material is strongly light scattering independent of thepolarization of incident light.

When the electric field is turned on to homeotropically align the liquidcrystal directors as shown in FIG. 2, the polymer-liquid crystallinematerial is optically clear. Because of the typically small amount ofpolymer in the composition, there is no significant variation orfluctuation of a refractive index throughout the liquid crystallinematerial. Therefore, the cell incorporating the material is haze-free atall viewing angles, although as the amount of polymer is increased theamount of haze may increase.

EXAMPLE 1

A normal mode cell, light scattering in the field-OFF condition andoptically clear in a field-ON condition, was prepared using a solutionof chiral nematic liquid crystal containing 8.8% by weight chiralmaterial based on the combined weight of nematic liquid crystal andchiral material, and 2.9% by weight of a cross-linking monomer based onthe combined weight of chiral nematic liquid crystal and monomer. Thechiral nematic liquid crystal had a pitch length of about 1.4 microns.

The polymerizable solution consisted of:

175.2 mg. of E-31LV nematic liquid crystal mixture;

8.3 mg. of 4"-(2-methylbutylphenyl)-4'-(2-methylbutyl)-4-biphenylcarboxylate (chiral material);

8.5 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

5.7 mg. of bisacryloyl biphenyl (BAB lab synthesized monomer);

1.0 mg. of benzoin methyl ether (BME photo-initiator).

E-31LV consists essentially of a mixture of: 4 ethyl 4'-cyanobiphenyl,4-butyl-4'-cyanobiphenyl, 4-hexyl-4'-cyanobiphenyl,4-methoxy-4'-cyanobiphenyl, 4-propyloxy-4'-cyanobiphenyl,4-pentyl-4'-cyanoterphenyl and 4-(4'-ethylbiphenyl-4-carbonyloxy) methylbenzene.

A cell having two glass plates sealed at the edges and separated by 8micron thick Mylar spacers was filled with the polymerizable solution.The glass plates were coated with indium-tin oxide to providetransparent electrodes. The electrodes were coated with polyimide andbuffed to affect homogeneous surface alignment of the liquid crystal.

The filled cell was irradiated with U.V. light to polymerize the monomerand cause phase separation of the polymer into polymer domains in thecell. While the cell was being irradiated, an AC electric voltage wasapplied to cause homeotropic alignment of the liquid crystal.

At zero field, the cell was opaque or light scattering and thetransmittance in this state was less than 5%. When an AC voltage(V_(rms) =35V) was applied across the cell, it became optically clear.In the field-ON condition, the cell was transparent for incident lightat all viewing angles.

EXAMPLE 2

The following series of normal mode cells were prepared with varyingpitch lengths and concentrations of chiral material.

A normal mode cell was prepared using a solution of chiral nematicliquid crystal containing 11.1% by weight chiral material based on thecombined weight of nematic liquid crystal and chiral material, and 2.9%by weight of a cross-linking monomer based on the combined weight ofchiral nematic liquid crystal and monomer. The chiral nematic liquidcrystal had a pitch length of about 1.4 microns.

The polymerizable solution consisted of:

144 mg. of E-31LV nematic liquid crystal mixture from EM Industries;

18 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

4.9 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

1.0 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared as in Example 1. An AC electric voltage wasapplied during polymerization to cause homeotropic alignment of theliquid crystal.

At zero field, the cell was opaque or light scattering and thetransmittance was less than 5%. When an AC voltage (V_(rms) =25V) wasapplied across the cell, it became optically clear. In the field-ONcondition, the cell was transparent for incident light at all viewingangles.

A normal mode cell was prepared using a solution of chiral nematicliquid crystal containing 12.9% by weight chiral material based on thecombined weight of nematic liquid crystal and chiral material and 3.3%by weight of a cross-linking monomer based on the combined weight ofchiral nematic liquid crystal and monomer. The chiral nematic liquidcrystal had a pitch length of about 1.2 microns.

The polymerizable solution consisted of:

137 mg. of E-31LV nematic liquid crystal mixture;

20.4 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

5.3 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

2.4 mg. of benzoin methyl ether (photo-initiator). The cell was preparedas in Example 1. An AC electric voltage was applied duringpolymerization to cause homeotropic alignment of the liquid crystal.

At zero field, the cell was opaque or light scattering and thetransmittance of the cell was 15%. When an AC voltage (V_(rms) =21V) wasapplied across the cell, it became optically clear. In the field-ONcondition, the cell was transparent for incident light at all viewingangles. Because the zero field transmittance was 15%, the cell wasmarginally effective.

A normal mode cell was prepared using a solution of chiral nematicliquid crystal containing 5.9% by weight chiral material based on thecombined weight of nematic liquid crystal and chiral material and 2.2%by weight of a cross-linking monomer based on the combined weight ofchiral nematic liquid crystal and monomer. The chiral nematic liquidcrystal had a pitch length of 2.6 microns.

The polymerizable solution consisted of:

200 mg. of E-31LV nematic liquid crystal;

12.6 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

4.8 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

1.8 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared as in Example 1. An AC electric voltage wasapplied during polymerization to cause homeotropic alignment of theliquid crystal.

At zero field, the cell was opaque or light scattering and thetransmittance of the cell was 62%. When an AC voltage (V_(rms) =14V) wasapplied across the cell, it became optically clear. In the field-ONcondition, the cell was transparent for incident light at all viewingangles. Because the zero field transmittance was 62%, the cell was notacceptable.

A normal mode cell was prepared from a polymerizable solution consistingof 0.96% R-1011 chiral material (E. Merck), 94.82% ZLI-4389 nematicliquid crystal mixture (E. Merck), 3.83% BAB6 lab synthesizedbisacrylate monomer, and 0.38% BME photo-initiator introduced betweenglass plates coated with ITO to serve as transparent electrodes. Theglass plates were cleaned but did not have any surface treatment foralignment of the liquid crystal. The thickness of the cell wascontrolled by 15 μm glass fiber spacers. The sample was irradiated withU.V. light to polymerize the monomer.

This sample had a pitch length of 3.9 μm. In the field-OFF condition thetransmittance of the sample was 1.8%. In the field-ON condition thetransmittance was 88%. The contrast ratio was 49:1. The sample had a lowdrive voltage of V_(rms) =8V. The turn-on and turn-off times were 21 msand 66 ms, respectively. This cell was acceptable.

Another normal mode cell was prepared just like the preceding cell froma polymerizable solution consisting of 2.16% R-1011 chiral agent, 93.36%ZLI-4389 nematic liquid crystal mixture, 3.83% BAB6 bisacrylate monomerand 0.38% BME photo-initiator. This sample had a pitch length of 1.74μm. In the field-OFF condition the transmittance of the cell was 0.37%.In the field-ON condition the transmittance was 87%. The cell had anexcellent contrast ratio of 230:1. The drive voltage was 15V and theturn-on and turn-off times were 50 ms and 14 ms, respectively.

Another normal mode cell was prepared just like the preceding examplefrom a polymerizable solution consisting of 2.62% R-1011 chiralmaterial, 92.62% ZLI-4389 nematic liquid crystal mixture, 4.38% BAB6bisacrylate monomer and 0.44% BME photo-initiator. This sample had apitch length of 1.4 μm. In the field-OFF condition the transmittance ofthe cell was 0.35%. In the field-ON condition the transmittance was 87%.This cell had an excellent contrast ratio of 250:1. The drive voltagewas 20V and the turn-on and turn-off times were 37 ms and 25 ms,respectively. It was noted that the drive voltage tends to increase asthe pitch length is decreased.

EXAMPLE 3

The following series of normal mode cells were prepared with varyingconcentrations of the monomer 4,4'-bisacryloyl biphenyl (BAB).

A normal mode cell was prepared using a solution of chiral nematicliquid crystal containing 10.0% by weight chiral material based on thecombined weight of nematic liquid crystal and chiral material, and 4.4%by weight of a cross-linking monomer based on the combined weight ofchiral liquid crystal and monomer. The chiral nematic liquid crystal hada pitch length of about 1.5 microns.

The polymerizable solution consisted of:

107 mg. of E-31LV nematic liquid crystal;

11.91 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

5.5 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

1.7 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared as in Example 1. An AC electric field was appliedduring polymerization to cause homeotropic alignment of the liquidcrystal.

At zero field, the cell was opaque or light scattering and thetransmittance was 7%. When an AC voltage (V_(rms) =28V) was appliedacross the cell, it became optically clear. In the field-ON condition,the cell was transparent for incident light at all viewing angles. Thiscell had good contrast between the field-ON and field-OFF states.

Another normal mode cell was prepared using a solution of chiral nematicliquid crystal containing 9.9% by weight chiral material based on thecombined weight of nematic liquid crystal and chiral material and 1.2%by weight of a cross-linking monomer based on the combined weight ofchiral nematic liquid crystal and monomer. The chiral nematic liquidcrystal had a pitch length of about 1.5 microns.

The polymerizable solution consisted of:

164 mg. of E-31LV nematic liquid crystal;

18 mg. 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

2.2 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

0.7 mg. benzoin methyl either (photo-initiator).

The cell was prepared as in Example 1. An AC electric field was appliedduring polymerization to cause homeotropic alignment of the liquidcrystal.

At zero field, the transmittance was 67%. When an AC voltage (V_(rms)=18V) was applied across the cell, it became optically clear. In thefield-ON condition, the cell was transparent for incident light at allviewing angles. Because the zero field transmittance was 67%, the cellwas not acceptable.

A normal mode cell was prepared using a solution of chiral nematicliquid crystal containing 9.8% by weight chiral material based on thecombined weight of nematic liquid crystal and chiral material and 9.0%by weight monomer based on the combined weight of chiral liquid crystaland monomer. The chiral liquid crystal had a pitch length of about 1.5microns.

The polymerizable solution consisted of:

101 mg. of E-31LV nematic liquid crystal;

11.0 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

11.4 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

3.5 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared as in Example 1. The cell was almost transparentat zero field and was not a good cell.

EXAMPLE 4

A normal mode cell without any surface aligning layer was prepared froma polymerizable solution consisting of 2.18% R-1011 chiral material (E.Merck), 94.14% ZLI-4389 nematic liquid crystal (E. Merck), 2.90% BAB6lab synthesized bisacrylate monomer, and 0.29% BME photo-initiator. Themixture was introduced between glass plates having ITO transparentelectrodes. The thickness of the cell was controlled by 15 μm glassspacers. The glass plates were cleaned but did not have any surfacetreatments for alignment of the liquid crystal. The sample wasirradiated with U.V. light to polymerize the monomer.

The sample required a drive voltage of 18V. The turn-ON time was 52 msand the turn-OFF time was 19 ms. The contrast ratio was 1:245 and thetransmission in the field-ON condition was 90%.

Reverse Mode Cells

Reverse mode cells which are optically clear in a field-OFF conditionand light scattering in a field-ON condition are prepared using a chiralnematic liquid crystal having a pitch length outside the visiblespectrum, preferably in the infrared spectrum. In the case of reversemode cells, the pitch length varies from about 1.0 to about 4.0 microns.The chiral nematic liquid crystal typically consists of from about 1 toabout 16% by weight chiral material based on the combined weight ofnematic liquid crystalline chiral material; although, the weight amountscan vary depending upon the particular liquid crystal, chiral materialand polymer which are used.

Reverse mode cells are made by polymerizing the polymer-liquid crystalsolution at zero field. As shown in FIG. 4, the liquid crystal moleculesthroughout the material prefer a twisted planar texture represented byreference numeral 40. A single polymer domain is again conceptuallyrepresented at 15. In the field-OFF condition shown in FIG. 4, apolymer-liquid crystalline material is optically clear, since there isno reflecting or scattering of light in the visible spectrum.

In the field-ON condition conceptually shown in FIG. 5, the liquidcrystal molecules in the vicinity of the polymer domains 15 prefer thetwisted planar orientation because of the anchoring affect of thepolymer domains. The surrounding liquid crystal is reformed by theelectric field to a focal conic texture. The focal conic texture in thepresence of the field is a result of competition between the forces inthe system such as any surface effects, the electric field and theconstraint of the polymer domains. In the field-ON condition of FIG. 5,the liquid crystalline-polymer material is strongly light scattering forall polarizations of incident light.

EXAMPLE 5

A reverse mode cell, light scattering in the field-ON condition andoptically clear in a field-OFF condition, was prepared using a solutionof chiral nematic liquid crystal containing 4.0% by weight chiralmaterial based on the combined weight of nematic liquid crystal andchiral material, and 4.6% by weight of a cross-linking monomer based onthe combined weight of chiral nematic liquid crystal and monomer. Thechiral nematic liquid crystal had a pitch length of about 3.7 microns.

The polymerizable solution consisted of:

107 mg. of E-31LV nematic liquid crystal;

4.5 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

5.5 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

1.2 mg. of benzoin methyl ether (photo-initiator).

A cell having two glass plates sealed at the edges and separated by 8micron thick Mylar spacers was filled with the polymerizable solution.The glass plates were coated with indium-tin oxide to providetransparent electrodes. The electrodes were coated with polyimide andbuffed to affect homogeneous surface alignment of the liquid crystal.The filled cell was irradiated with U.V. light to polymerize the monomerand cause phase separation of the polymer into polymer domains in thecell.

At zero field, the cell was optically clear and transparent for incidentlight at all viewing angles. In the field-OFF state the transmittancewas 93%. When an AC voltage (V_(rms), =27V) was applied across the cell,it became opaque or light scattering. In the field-ON state, thetransmittance was 8%, independent of the polarization of the incidentlight.

EXAMPLE 6

A series of reverse mode cells were made with varying pitch lengths andconcentrations of chiral materials.

A reverse mode cell was prepared using a solution of chiral nematicliquid crystal containing 7.8% by weight chiral material based on theweight of nematic liquid crystal and chiral material and 4.7% by weightof a cross-linking monomer based on the weight of chiral liquid crystaland monomer. The chiral nematic liquid crystal had a pitch length ofabout 1.9 microns.

The polymerizable solution consisted of:

159 mg. of E-31LV nematic liquid crystal;

13.7 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

8.5 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

0.8 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared in the manner described in Example 5.

At zero field, the cell was optically clear and transparent for incidentlight at all viewing angles. In the field-Off state the transmittancewas 98%. When an AC voltage (V_(rms) =30V) was applied, the cell becameopaque or light scattering. In the field-ON state, the transmittance was9% and was independent of the polarization of the incident light.

A reverse mode cell was prepared using a solution of chiral nematicliquid crystal containing 12.1% by weight chiral material based on thecombined weight of nematic liquid crystal and chiral material and 4.8%by weight of a cross-linking monomer based on the combined weight ofchiral nematic liquid crystal and monomer. The chiral nematic liquidcrystal had a pitch length of about 1.3 microns.

The polymerizable solution consisted of:

129 mg. of E-31LV nematic liquid crystal;

17.7 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

7.5 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

0.6 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared in the manner described in Example 5.

At zero field, the cell was optically clear and transparent for incidentlight at all viewing angles. In the field-OFF state the transmittancewas 96%. When an AC voltage (V_(rms) =41V) was applied, the cell becameopaque or light scattering. In the field-ON state, the transmittance was5%, independent of the polarization of the incident light.

A reverse mode cell was prepared using a solution of chiral nematicliquid crystal containing 1.39% by weight chiral material based on thecombined weight of nematic liquid crystal and chiral material and 4.5%by weight of a cross-linking monomer based on the combined weight ofmonomer and chiral liquid crystal. The chiral nematic liquid crystal hada pitch length of about 10.7 microns.

The polymerizable solution consisted of:

263 mg. of E-31LV nematic liquid crystal;

3.7 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

12.5 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

1.4 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared in the manner described in Example 5.

At zero field, the cell was optically clear and transparent for incidentlight at all viewing angles. In the field-OFF state the transmittancewas 92%. When an AC voltage (V_(rms) =32V) was applied, thetransmittance of polarized light parallel to the buffing direction was35%. When an AC voltage (V_(rms) =18V) was applied, the transmittance ofpolarized light perpendicular to the buffing direction was 6%.

A reverse mode cell was prepared using a solution of chiral nematicliquid crystal containing 16.3% by weight chiral material based on theweight of nematic liquid crystal and chiral material and 4.4% by weightof a cross-linking monomer based on the weight of monomer and liquidcrystal. The chiral nematic liquid crystal had a pitch length of about0.9 microns.

The polymerizable solution consisted of:

125.7 mg. of E-31LV nematic liquid crystal;

24.4 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

6.9 mg. 4,4'-bisacryloyl biphenyl (monomer); and

0.7 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared in the same manner as Example 5.

At zero field, the cell was optically clear and transparent for incidentlight at all viewing angles. In the field-Off state the transmittancewas 94%. When an AC voltage (V_(rms) =50V) was applied, the cell becameopaque or light scattering. In the field-ON state, the transmittance was11% and was independent of the polarization of the incident light. Thiscell also displayed an hysterisis effect as the cell was switched fromthe field-ON to field-OFF state.

EXAMPLE 7

The following series of reverse mode cells were prepared with varyingpolymer concentrations.

A reverse mode cell was prepared using a solution of chiral nematicliquid crystal containing 6.1% by weight chiral material based on thecombined weight of chiral material and nematic liquid crystal and 2.1%by weight of a cross-linking monomer based on the combined weight ofmonomer and chiral nematic liquid crystal. The chiral nematic liquidcrystal had a pitch length of about 2.5 microns.

The polymerizable solution consisted of:

181 mg. of E-31LV nematic liquid crystal;

11.7 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

4.2 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

0.9 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared in the manner described in Example 5.

At zero field, the cell was optically clear and transparent for incidentlight at all viewing angles. In the field-OFF state the transmittancewas 97%. When an AC voltage (V_(rms) =18V) was applied, the cell becameopaque or light scattering. In the field-ON state, the transmittance was14%, and independent of the polarization of the incident light. Theturn-off time for this cell was 5 milliseconds.

A reverse mode cell was prepared using a solution of chiral nematicliquid crystal material containing 6% by weight chiral material based onthe combined weight of nematic liquid crystal and chiral material, and2.9% by weight of a cross-linking monomer based on the combined weightof chiral nematic liquid crystal and monomer. The chiral nematic liquidcrystal has a pitch length of about 2.5 microns.

The polymerizable solution consisted of:

181 mg. of E-31LV nematic liquid crystal;

11.5 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

5.7 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

0.7 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared in the same manner described in Example 5.

At zero field, the cell was optically clear and transparent for incidentlight at all viewing angles. In the field-OFF state the transmittancewas 95%. When an AC voltage (V_(rms) =21V) was applied, the cell becametranslucent or light scattering. In the field-ON state, thetransmittance was 8% and independent of the polarization of the incidentlight. The turn-off time for the cell was 4 milliseconds.

A reverse mode cell was prepared using a solution of chiral nematicliquid crystal containing 6.0% by weight chiral material based on thecombined weight of the nematic liquid crystal and chiral material, and4.3% by weight of a cross-linking monomer based on the combined weightof monomer and chiral liquid crystal. The chiral nematic liquid crystalhas a pitch length of about 2.5 microns.

The polymerizable solution consisted of:

180 mg. of E-31LV nematic liquid crystal;

11.5 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

8.6 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

0.9 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared in the manner described in Example 5.

At zero field, the cell was optically clear and transparent for incidentlight at all viewing angles. In the field-OFF state the transmittancewas 95%. When an AC voltage (V_(rms) =28V) was applied, the cell becametranslucent or light scattering. In the field-ON state, thetransmittance was 8% and independent of the polarization of the incidentlight. The turn-off time of the cell was 3 milliseconds.

A reverse mode cell was prepared using a solution of chiral nematicliquid crystal containing 6.1% by weight chiral material based on thecombined weight of nematic liquid crystal and chiral material, and 5.8%by weight of a cross-linking monomer based on the combined weight ofchiral liquid crystal and monomer. The chiral nematic liquid crystal hada pitch length of about 2.5 microns.

The polymerizable solution consisted of:

179 mg. of E-31LV nematic liquid crystal;

11.7 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

10.6 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

1.2 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared in the manner described in Example 5.

At zero field, the transmittance was 76%. When the AC voltage (V_(rms)=28V) was applied across the cell, the transmittance decreased. In thefield-ON state, the transmittance was 46%.

A reverse mode cell was prepared using a solution of chiral nematicliquid crystal containing 6.0% by weight chiral material based on thecombined weight of nematic liquid crystal and chiral material and 1.0%by weight of a cross-linking monomer based on the combined weight of thechiral nematic liquid crystal and monomer. The chiral nematic liquidcrystal had a pitch length of about 2.5 microns.

The polymerizable solution consisted of:

184 mg. of E-31LV nematic liquid crystal;

11.7 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

20 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

0.7 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared in the manner described in Example 5.

At zero field, the cell was optically clear and transparent for incidentlight at all viewing angles. In the field-OFF state the transmittancewas 96%. When an AC voltage (V_(rms) =14V) was applied across the cell,the cell became translucent or light scattering. In the field-ON state,the transmittance was 13%, independent of the polarization of theincident light. The turn-off time was 20 milliseconds, which is long.The cell was also easily damaged by the electric field.

EXAMPLE 8

A reverse mode cell was prepared as in the preceding examples consistingof two glass plates separated by 10 μm spacers filled with apolymerizable solution consisting of 94.53% ZLI-4389 nematic liquidcrystal (E. Merck), 1.25% ZLI-4572 chiral material (E. Merck), 3.83BP6DA lab synthesized monomer, and 0.38% photo-initiator. The glassslides were coated with ITO electrodes and rubbed polyimide forhomogeneous surface alignment of the liquid crystal. The material wasthen polymerized by U.V. irradiation for 2 hours.

In the field-OFF condition the transmittance was 85%. Upon applicationof a field the transmittance decreased, reaching a minimum transmittanceof 10% at 15 volts. When the applied voltage was decreased the materialreturned to the original transmitting state.

EXAMPLE 9

A reverse-mode cell was prepared without a polyimide alignment layer asa surface treatment on the cell walls. The polymer stabilized liquidcrystal material was prepared from 94.54% ZLI-4389 nematic liquidcrystal mixture, 1.25% ZLI-4572 chiral agent, 3.83% lab synthesizedbisacrylate monomer BP6DA, and 0.38% BME photo-initiator and introducedbetween two glass slides separated by 15 μm glass fiber spacers as inthe previous examples. The glass slides were coated with ITO electrodes,but had no other surface treatment except cleaning. The cell was filledby capillary action in a vacuum chamber. The sample was irradiated by UVlight for 2 hours to polymerize the monomer.

In the field-OFF condition the transmittance was 18%. When an externalelectric voltage was applied to the cell, the transmittance of the celldecreased. A minimum value of 2% was reached when the voltage wasincreased to 20V. When the voltage was decreased, the materialtransformed back to its original texture and the transmittance of thecell increased, although the dynamic response of the cell wascomparatively slow.

Multistable Color Display Cells

The multistable color display material of the invention exhibits astable grey scale phenomenon characterized by the ability of thematerial to reflect indefinitely any selected intensity of light betweenthe intensity reflected by the reflecting state and that reflected bythe scattering state. When the material is in the reflecting state thechiral material assumes a planar texture which reflects colored light ata maximum intensity for a given material, the color of the reflectedlight being determined by the pitch length of the chiral material. Anelectric field pulse of an appropriate threshold voltage, typically inthe range of about 4 to 5 volts per micrometer of thickness, will causeat least a portion of the material to change its optical state and theintensity of reflectivity to decrease. If the AC pulse is high enough,e.g., in the range of about 6 to 8 volts per micrometer of thickness,the optical state of the material will change completely to thescattering state in which the chiral material exhibits a focal conictexture which reflects light at a minimum intensity for a givenmaterial. In between the reflecting state, which for a given materialcan be considered to define the maximum intensity of reflectivity forthat material, and the scattering state, which can be considered todefine the minimum intensity of reflectivity, the intensity ofreflectivity ranges along a grey scale, which is simply a continuum ofintensity values between that exhibited by the reflecting and scatteringstates. By pulsing the material with an AC pulse of a voltage below thatwhich will convert the material from the reflecting state to thescattering state, or visa versa, one obtains an intensity ofreflectivity in this grey scale range.

While not wanting to be bound by theory, it has been observed that theintensity of reflectivity along the grey scale is approximately linearlyproportional to the voltage of the pulse. By varying the voltage of thepulse the intensity of reflectivity of a given color can be variedproportionally. When the electric field is removed the material willreflect that intensity indefinitely. It is believed that pulses withinthis grey scale voltage range cause a proportion of the material toconvert from the planar texture characteristic of the reflecting state,to the focal conic texture characteristic of the scattering state. Sinceboth the planar texture of the reflecting state and the focal conictexture of the scattering state are stabilized by the polymer in thezero field condition, the grey scale intensities reflected by thedisplay are also stable since the material in these optical statessimply comprises a combination of both the stable planar texture and thestable focal conic texture. The intensity of reflectivity along the greyscale is proportional to the amount of chiral material switched from theplanar texture to the focal conic texture, or vise versa, which is inturn proportional to the voltage of the AC pulse.

Multistable color display cells which scatter light in one state andreflect circularly polarized colored light in another state with stablegrey scale reflection therebetween, and which also can be operated toexhibit optical transparency, are made using chiral nematic liquidcrystal which has a pitch length effective to reflect light in thevisible spectrum. Preferred materials have a pitch length ranging fromabout 0.25 to about 0.44 microns. Typical pitch lengths are 0.27 micronsfor blue color, 0.31 microns for green colors and 0.40 microns for redcolor. Multistable color display materials have been prepared to containfrom about 27 to about 66% chiral material based on the combined weightof nematic liquid crystal and chiral material; although, as in the caseof previously described embodiments, the weight amount can varydepending upon the particular chiral material, nematic liquid crystaland polymer which are used.

FIG. 4 conceptually illustrates a single polymer domain 15 of themultistable color display material of the invention in its lightreflecting state. In this state, the chiral liquid crystal molecules 40are oriented in a twisted planar structure parallel to the cell walls.Because of the twisted planar texture the material will reflect light,the color of which depends upon the particular pitch length. In thisstable reflecting state, the material exhibits maximum reflectivity thatconstitutes a maximum reference intensity below which the grey scaleintensities are observed.

The planar texture of the liquid crystal in the vicinities of thepolymer domains 15 is stabilized by the polymer. The surrounding liquidcrystal indicated by reference numeral 50 in FIG. 5, being lessstabilized, tends to reform to the focal conic texture when an ACvoltage pulse is applied to the cell. As conceptually illustrated inFIG. 5, the multistable color display material is in its lightscattering state. In this stable scattering state the material exhibitsits minimum intensity of reflection (i.e., maximum scattering) whichdefines a minimum reference intensity of reflectivity above which thegrey scale intensities are observed.

If the pitch length of the polymer stabilized liquid crystal material isin the range effective to reflect visible light, both the lightreflecting state of FIG. 4 and the light scattering state of FIG. 5, aswell as the grey scale states therebetween, are stable in the absence ofan electric field. If the multistable material is in the lightreflecting state of FIG. 4 and a low electric field pulse is applied,for example, about 6 volts per micron, the material will be driven tothe light scattering state of FIG. 5 and will remain in that state atzero field. If the multistable material is in the light scattering stateof FIG. 5 and a higher electric field pulse sufficient to untwist thechiral molecules is applied, e.g., about 10 volts per micron ofthickness, the liquid crystal molecules will reform to the lightreflecting state of FIG. 4 at the end of the pulse and will remain inthat condition. It is to be understood that the voltages per micronnecessary to drive the material between optical states may varydepending on the composition of the material, but that the determinationof necessary voltages is well within the skill in the art in view of theinstant disclosure.

If the pitch length of the liquid crystal material is in the rangeeffective to reflect light in the ultra violet range, a variant ofmultistable cell can be prepared which functions as a bistable lightshutter. When the material is in the stable planar texture the cellappears clear because the light reflected from the cell is outside thevisible spectrum. As with the color reflecting cells, the material willscatter light when switched to the stable focal conic texture. Hence,the multistable material-can be switched between a stable opticallyclear state, where the liquid crystal reflects light in the ultra violetrange, and a stable light scattering state. Pitch lengths effective toreflect light in the ultra violet range will typically be from about 0.5to about 1 micron. Bistable light shutters that reflect light in theultra violet, and hence appear clear in the planar texture, and scatterlight in the focal conic texture have been prepared containing about 18%chiral material based on the combined weight of chiral material andnematic liquid crystal.

If the high electric field necessary to untwist the liquid crystalmolecules in the multistable color display materials is maintained, theliquid crystal directors will be homeotropically aligned so that thematerial is transparent. If the field is slowly removed, the liquidcrystal orientation will reform to the light scattering state of FIG. 5.When the field is quickly removed, the orientation will reform to thelight reflecting state of FIG. 4. The intensities of reflectivityreflected between the reflecting state of FIG. 4 and the scatteringstate of FIG. 5 are stable grey scale reflectivities. Of course, theintensity value of the reflecting and scattering states may vary as thecomposition of the material varies, but the grey scale is defined by therange of intensities therebetween.

At voltages less than that which will transform the material from thereflecting state of FIG. 4 to the scattering state of FIG. 5, grey scalestates which are themselves stable at zero field are obtained. Thereflection from the material in these grey scale states is stablebecause a proportion of the material is in the planer reflecting textureof FIG. 4 and a proportion of the material is in the focal conicscattering texture of FIG. 5, both of which are stabilized by thepolymer in the absence of a field.

Thus, for example, if the material is in the reflecting state of FIG. 4and an electric field pulse is applied having a voltage insufficient todrive all of the liquid crystal 16 surrounding the polymer domains 15into the focal conic texture shown at 50 in FIG. 5, i.e., insufficientto drive the material completely to the scattering state, the materialwill reflect colored light of an intensity that is proportional to theamount of material that remains in the planar reflecting texture. Thereflectivity will thus be lower than that reflected from the materialwhen all of the chiral material is in the planar reflecting texture, butstill higher than when switched completely to the focal conic scatteringtenure. As the voltage of the electric field pulse is increased, more ofthe chiral material is switched from the planar reflecting texture tothe scattering focal conic texture and the reflectivity decreasesfurther until the voltage of the pulse is increased to the point wherethe material is completely switched to the scattering state. If thevoltage of the pulse is increased still further, the intensity ofreflection begins to increase again until the magnitude of the pulse issufficient to untwist the chiral molecules so that they will againreform to the planar light reflecting texture when the pulse is removedand the material is again in the light reflecting state of FIG. 4.

If the material is in the focal conic scattering state of FIG. 5, anapplied electric field pulse will have a negligible effect on thereflectivity of the cell until it reaches a magnitude sufficient tountwist the chiral material, whereby it will reform to the lightreflecting state of FIG. 4, as described above, when the field isremoved. The grey scale response of a cell as described above isillustrated in FIG. 6 which shows the response of the material preparedin Example 10 to varying pulse voltages.

EXAMPLE 10

A multistable grey scale display cell was made from a polymer stabilizedchiral nematic liquid crystalline material of the following components:

160.7 mg--CB15 cholesteric liquid crystal, BDH Chemicals

160.7 mg--CE2 cholesteric liquid crystal, BDH Chemicals

488.8 mg--E31 nematic liquid crystal, BDH Chemicals

8.0 mg--BAB (4,4'-bisacryloylbiphenyl), lab synthesized monomer

3.0 mg--BME (benzoinmethyl ether), Polyscience Co., photo-initiator

2.2 mg--R4, dichroic dye

A mixture of the liquid crystal and monomer was sandwiched between twoglass plates with ITO electrodes. The glass plates were polyimide coatedand buffed for homogeneous alignment of the liquid crystal. The backplate was painted black and separated from the front plate by 5 μm glassfibers. In the reflecting state the cell reflected green color. In thescattering state the cell was black. The filled cell was irradiated withU.V. light for thirty minutes to polymerize the monomer and cause phaseseparation of the polymer into phase separated polymer domains in thecell.

The reflectivity of the cell in response to AC pulse of varying voltageswas measured. In the measurement, square AC pulses of width of 10milliseconds (ms) were used. For this material an applied pulse of 34Vswitched the cell completely into the scattering state, independent ofwhether it was in the reflecting state or the scattering state beforethe pulse. Minimum reflection is observed here. An applied pulse of 50Vswitched the cell into the reflecting state independent of whether thecell was in the scattering or reflecting state prior to the pulse.Maximum reflection is observed here. The transformation from thereflecting to the scattering state was near 0.5 ms. The transformationfrom the scattering to the reflecting state was near 300 ms.

The grey scale response of the cell in response to pulses of varyingvoltage is seen in FIG. 6. Here the voltage of the pulse was varied andthe reflection of the cell was measured one second after the pulse.Curve A is the response of the cell when the material is in thereflecting state prior to each pulse. Prior to each pulse plotted oncurve A the material was subjected to a high AC pulse of about 50V toensure that it was completely in the reflecting state prior to thepulse. When the voltage of the pulse is below 20V, the reflection of thecell is not affected. When the voltage of the pulse is between 20V and34V, the later being the voltage necessary to switch the cell to thescattering state, the reflectivity of the cell decreases approximatelylinearly as the voltage of the pulse is increased. Grey scalereflectivity is observed in this voltage range. In each case thematerial continued to reflect after the pulse was removed. When thevoltage of the pulse was increased above 34V, the reflectivity of thecell increased until the reflectivity reached its original value, i.e.,that of the reflecting state, above 46V. Curve B is the response of thecell when the material was initially in the scattering state prior tothe AC pulse. Here the reflectivity of the cell remains unchanged for anAC pulse below 40V. Above 40V the material switched to the reflectingstate.

EXAMPLE 11

A color display cell reflecting red circularly polarized light in thereflecting state was prepared using a solution of chiral nematic liquidcrystal containing 29.8% by weight chiral material based on the combinedweight of chiral material and nematic liquid crystal, and 2.7% by weightof a cross-linking monomer based on the combined weight of monomer andchiral liquid crystal. The chiral liquid crystal had a pitch length of0.41 microns.

The polymerizable solution consisted of:

67.8 mg. of E-31LV nematic liquid crystal;

14.0 mg. of 4"-(2-methylbutylphenyl)-4'-(2-methylbutyl)-4-biphenylcarboxylate (chiral material);

14.8 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

2.7 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

1.0 mg. of benzoin methyl ether (photo-initiator).

A cell having two glass plates sealed at the edges and separated by 8micron thick Mylar spacers was filled with the polymerizable solution.The glass plates were coated with indium-tin oxide to providetransparent electrodes. The electrodes were coated with polyimide andbuffed to affect homogeneous surface alignment of the liquid crystal.

The filled cell was irradiated with U.V. light for thirty minutes topolymerize the monomer and cause phase separation of the polymer intophase separated polymer domains. Upon the application of a high ACvoltage pulse, about 104V, the cell was optically clear and transparentto incident light. When the pulse was removed the cell was in the stableplanar reflecting state. An AC pulse between about 50 and 85V switchedthe cell to a stable focal conic scattering state. Both the reflectingand scattering states were stable at zero field for over several months.

EXAMPLE 12

A cell having a blue reflecting state was prepared using a solution ofchiral nematic liquid crystal containing 45.3% by weight chiral materialbased on the weight of chiral material and nematic liquid crystal, and1.5% by weight of a cross-linking monomer based on the combined weightof monomer and chiral nematic liquid crystal. The chiral nematic liquidcrystal had a pitch length of about 0.27 microns, reflecting bluecircularly polarized light in the reflecting state.

The polymerizable solution consisted of:

132.6 mg. of E-31LV nematic liquid crystal;

50.0 mg. of 4"-(2-methylbutylphenyl)-4'-(2-methylbutyl)-4-biphenylcarboxylate (chiral material);

59.7 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

3.7 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

1.0 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared as in Example 11. The cell was in a bluereflecting state after the removal of the high voltage pulse.

EXAMPLE 13

A cell having a green reflecting state was prepared using a solution ofchiral nematic liquid crystal containing 39.1% by weight chiral materialbased on the weight of chiral material and nematic liquid crystal and2.0% by weight of a cross-linking monomer based on the combined weightof chiral nematic liquid crystal and monomer. The chiral liquid crystalhad a pitch length of about 0.31 microns, reflecting green circularlypolarized light.

The polymerizable solution consisted of:

85.6 mg. of E-31LV nematic liquid crystal;

27.0 mg. of 4"-(2-methylbutylphenyl)-4'-(2-methylbutyl)-4-biphenylcarboxylate (chiral material);

28.0 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

2.9 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

1.0 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared as in the preceding examples. The cell was in agreen reflecting state after the removal of the high voltage pulse.

EXAMPLE 14

A cell having a red reflecting state was prepared using a solution ofchiral nematic liquid crystal containing 30.0% by weight chiral materialbased on the combined weight of chiral material and nematic liquidcrystal and 1.9% by weight of a cross-linking monomer based on thecombined weight of chiral nematic liquid crystal and monomer. The chiralliquid crystal has a pitch length of about 0.41 microns.

The polymerizable solution consisted of:

80.0 mg. of E-31LV nematic liquid crystal;

16.7 mg. of 4"-(2-methylbutylphenyl)-4'-(2-methylbutyl)-4-biphenylcarboxylate (chiral material);

17.5 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

22 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

1.0 mg. of benzoin methyl ether (photo-initiator).

The sample cell was prepared as in the preceding examples. The cell wasa red reflecting state after removal of the high voltage pulse.

EXAMPLE 15

A green reflecting cell with a greater degree of contrast between thereflecting state and scattering state was prepared using a solution ofchiral nematic liquid crystal containing 39.1% by weight chiral materialbased on the combined weight of chiral material and nematic liquidcrystal and 2.0% by weight of a cross-linking monomer based on thecombined weight of monomer and chiral nematic liquid crystal. The chiralnematic liquid crystal had a pitch length of about 0.31 microns,reflecting green circularly polarized light.

The polymerizable solution consisted of:

85.6 mg. of E-31LV nematic liquid crystal;

27.0 mg. of 4"-(2-methylbutylphenyl)-4'-(2-methylbutyl)-4-biphenylcarboxylate (chiral material);

28.0 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

2.7 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

1.0 mg. of benzoin methyl ether (photo-initiator).

A cell having two glass plates sealed at the edges and separated by 8micron thick Mylar spacers was filled with the polymerizable solution.The glass plates were coated with indium-tin oxide to providetransparent electrodes. The electrodes were coated with polyimide andbuffed to affect homogeneous surface alignment of the liquid crystal.

The filled cell was irradiated with U.V. light to polymerize the monomerand cause phase separation of the polymer into polymer domains. Whilethe cell was being irradiated, an AC electric voltage was applied tocause homeotropic alignment of the liquid crystal.

The state of the cell was controlled by the voltage of an electricpulse. When a high AC voltage (V_(rms) =104V) was applied, the cell wasoptically clear and transparent to all angles of incident light. Whenthe high AC voltage was removed, the sample was in the reflecting stateand because of the pitch of the chiral liquid crystal, the color of thecell was green. When an AC voltage (50V ≦V_(rms) ≦85V) was applied thecell switched to the light scattering state and after removal of the lowvoltage field the cell remained in the light scattering state. Both thereflecting and scattering states were observed to be stable states. Thiscell appeared to have better contrast between the reflecting andscattering states than the cell prepared in the preceding examples. Thisshould result in a broader grey scale obtainable by this material.

EXAMPLE 16

A cell was prepared where no monomer was added to the chiral nematicliquid crystal. The solution of chiral nematic liquid crystal contained32.4% by weight chiral material based on the combined weight of chiralmaterial and nematic liquid crystal.

The solution consisted of:

121.6 mg. of E-31LV nematic liquid crystal;

29.7 mg. of 4"-(2-methylbutylphenyl)-4'-(2-methylbutyl)-4-biphenylcarboxylate (chiral material); and

20.5 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material).

A cell having two glass plates sealed at the edges and separated by 10micron thick Mylar spacers was filled with the chiral nematic liquidcrystal solution. The glass plates were coated with indium-tin oxide toprovide transparent electrodes. The electrodes were coated withpolyimide and buffed to affect homogeneous surface alignment of theliquid crystal.

At zero field, the scattering state changed into the reflecting stateover a time span of one hour. The reflecting state of this cell was notuniform as well.

EXAMPLE 17

A multistable cell was prepared using a solution of chiral nematicliquid crystal containing 29.8% by weight chiral material based on thecombined weight of chiral material and nematic liquid crystal and 2.7%by weight of a cross-linking monomer based on the combined weight ofchiral nematic liquid crystal and monomer. The chiral nematic liquidcrystal had a pitch length of about 0.41 microns.

The polymerizable solution consisted of:

67.8 mg. of E-31LV nematic liquid crystal;

14.0 mg. of 4"-(2-methylbutylphenyl)-4'-(2-methylbutyl)-4-biphenylcarboxylate (chiral material);

14.8 mg. of 4-cyano-4'-(2-methyl)-butylbiphenyl (chiral material);

2.7 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

1.0 mg. of benzoin methyl ether (photo-initiator).

The cell was prepared in the manner described in Example 11. The cell inthe reflecting state was red after the removal of the high voltagepulse. The voltages required to switch the cell to the reflecting orscattering states were the same as in Example 11. The reflecting stateand scattering state had good contrast and both states were stable.

A multistable cell was prepared using a solution of chiral nematicliquid crystal containing 30.3% by weight chiral material based on thecombined weight of chiral material and nematic liquid crystal and 6.9%by weight of a cross-linking monomer. The chiral nematic liquid crystalhad a pitch length of about 0.40 microns.

The polymerizable solution consisted of:

92.8 mg. of E-31LV nematic liquid crystal;

20.0 mg. of 4"-(2-methylbutylphenyl)-4'-(2-methylbutyl)-4-biphenylcarboxylate (chiral material);

20.3 mg. of 4-cyano-4'-(2-methyl) butylbiphenyl (chiral material);

9.9 mg. of 4,4'-bisacryloyl biphenyl (monomer); and

0.8 mg. of benzoin methyl ether (photo-initiator).

The sample cell was prepared in the manner described in Example 11. Thecell did not have a scattering state. Following removal of any ACelectric pulse, the sample cell returned to the reflective state. Thecolor of the cell in the reflective state was red.

EXAMPLE 18

A cell according to the invention was prepared using an epoxy. The epoxywas composed of, by weight, 6.1% Epon 828, 22.5% Capcure 3-800, 11.4%Heloxy 97. The epoxy was combined with 60% liquid crystal composed of amixture of 57.1% E7 (nematic liquid crystal mixture), 21.4% CB15 (chiralmaterial), and 21.5% CE2 (chiral material). The materials were handmixed for 5 minutes. A homogeneous solution was formed by heating themixture to 90° C. and stirring. The resulting mixture was sandwichedbetween ITO coated glass plates with 10 μm spacers. The resultingassembly was cured in a 90° C. oven overnight. The resulting shutter canbe switched from a focal conic to planar texture by application of a 175volt pulse and then back to the focal conic texture by application of an80 volt pulse. Both the planar and focal conic texture are stable withmultiple stable reflecting states, i.e., stable grey scale,therebetween. Advantageously, this film is self supporting and selfadhering in that the material itself seals the plates together.

EXAMPLE 19

Another light modulating cell was prepared from an optical adhesive U.V.cured polymer. A liquid crystal mixture comprising 57.1% E7, 21.4% CB15,and 21.5% CE2 was mixed with 25% by weight Norland Optical AdhesiveNOA65. The solution was heated to about 100° C. to form a homogeneoussolution of liquid crystal and polymer. The resulting mixture wassandwiched between two transparent conducting glass substrates with 20μm spacers to control thickness and then cured by U.V. light for 5minutes under an Oriel 450 watt Xenon arc lamp. The sandwich was kept ina 100° C. oven until immediately prior to U.V. curing. Upon theapplication of an appropriate field the resulting shutter was switchedbetween stable focal conic and planar textures, respectively, withmultiple stable grey scale states therebetween. As with the precedingexample, this film was also advantageously self supporting.

EXAMPLE 20

Several additional cells were prepared using various mixtures of E48 asthe nematic liquid crystal. In these cells the chiral material wascomprised of TM74A or various mixtures of CB15 and CE2 in amountsranging from 10 to 45% of the liquid crystal component. The polymerAU1033 (a hydroxy functionalized polymethacrylate) was used in thesecells in amounts ranging from 20-30% by weight based on the combinedweight of the liquid crystal mixture and polymer. A diisocyanatecrosslinking agent (N75) was added to the solutions of liquid crystaland polymer and the solutions incorporated into a 10 μm thick cells.Upon evaporation of the solvent the materials were thermally crosslinked at 100° C. The resulting shutters were all in a stable planartexture and could be switched to a stable focal conic texture upon theapplication of a field. Application of a higher field switched theshutters back to the planar texture.

Additional cells were made using E7 and E31 as the nematic host with 45%chiral additive selected from TM74A or mixtures of CE2 and CB15 and 30%AU1033 as the polymer binder. These shutters were also switchablebetween stable planar and focal conic textures.

EXAMPLE 21

More shutters were prepared using a chiral nematic liquid crystalcomprised of various mixtures of E48, CB15 and CE2, and TM74A in 5 and10% PMMA polymer. The mixtures were poured onto conducting glasssubstrates and the solvent allowed to evaporate. Spacers were used toproduce a final film thickness of 10 μm. The film was heated to 100° C.and a second substrate was added to form the cell. The resultingshutters reflected blue light in the planar reflecting texture. As withthe other shutters, the materials switched to a stable focal conictexture upon application of a field and then back to a stable planartexture upon application of a higher field. These cells were alsoadvantageously self sustaining.

EXAMPLE 22

A multistable color reflecting cell was prepared as in the precedingexamples from a material of the following components:

160.7 mg--CB15 cholesteric liquid crystal, BDH Chemicals

160.7 mg--CE2 cholesteric liquid crystal, BDH Chemicals

488.8 mg--E31 nematic liquid crystal, BDH Chemicals

8.0 mg--BAB (4,4'-bisacryloylbiphenyl), lab synthesized monomer

3.0 mg--BME (benzoinmethyl ether), Polyscience Co., photo-initiator

2.2 mg--R4, dichroic dye

The material was irradiated with U.V. light for thirty minutes topolymerize the monomer and cause phase separation of the polymer intopolymer domains in the cell. In the reflecting state the materialreflected green color. In the scattering state the material appearsblack when a black absorbing layer is used on the back substrate.

EXAMPLE 23

A multistable color reflecting display cell was made with the followingmaterials:

15.8% CE2 cholesteric liquid crystal, BDH Chemicals.

14.8% CB15 cholesteric liquid crystal, BDH Chemicals.

68.2% ZLI-4389 nematic liquid crystal, EM Industries, Inc.

1.2% DSM DesSolite 950-044 uv material, DSM Desotech Inc.

A cell was prepared as in the previous examples. The DesSolite is amixture of monomer, oligomer and photo-initiator. In the reflectingstate the cell reflects green color.

EXAMPLE 24

A bistable light shutter was prepared where the pitch length of theliquid crystal was adjusted to reflect light in the ultra violet range.A mixture of 6.3% CE2 cholesteric liquid crystal, 6.2% CB15 cholestericliquid crystal, 6.3% R-1011 chiral agent, 80% ZLI-4469-00 nematic liquidcrystal mixture, 1% BAB bisacrylate monomer and 0.1% BME photo-initiatorwas introduced between glass plates coated with ITO to serve astransparent electrodes. The glass plates were coated with polyimide andbuffed to produce homogeneous alignment of the liquid crystal. Thesample was irradiated by U.V. light to polymerize the monomer and formphase separated polymer domains in the cell. The cell exhibited a stableoptically clear state when switched to the planar texture and a stablescattering state when switched to the focal conic texture.

Many modifications and variations of the invention will be apparent tothose skilled in the art in light of the foregoing detailed disclosure.Therefore, within the scope of the appended claims, the invention can bepracticed otherwise than as specifically shown and described.

What is claimed:
 1. A method of selectively adjusting the intensity ofreflection of colored light from a light modulating material of chiralnematic liquid crystal and polymer, between a maximum and a minimumintensity comprising subjecting said material to varying electric fieldpulses of sufficient duration and voltage to cause a first proportion ofsaid chiral nematic material to exhibit a first optical state and asecond proportion of said chiral nematic material to exhibit a secondoptical state, whereby said material will continuously reflect aselected intensity between said maximum and minimum that is proportionalto the magnitude of the electric field pulse wherein said chiral nematicmaterial in said first optical state exhibits a reflective planartexture and said chiral nematic material in said second optical stateexhibits a transparent focal conic texture wherein both optical statesare stable in the absence of a field.
 2. The method according to claim 1comprising applying square A.C. voltage pulses.
 3. The method accordingto claim 2 comprising applying A.C. voltage pulses at a magnitude belowthat which will switch said material from said first optical state tosaid second optical state.
 4. A method of addressing a light modulatingcell comprising polymer and chiral nematic liquid crystalline lightmodulating material having positive dielectric anisotropy and a pitchlength effective to reflect light in the visible spectrum, cell wallstructure cooperating with said liquid crystal to form focal conic andtwisted planar textures that are stable in the absence of a field, andmeans for addressing said liquid crystal material, said methodcomprising selectively applying voltage pulses to said material of amagnitude effective to transform at least a portion of said liquidcrystal from a transparent light scattering focal conic texture to alight reflecting twisted planar texture, or to transform at least aportion of said liquid crystal from a light reflecting twisted planartexture to a transparent light scattering focal conic texture, wherebythe amount of transformation is dependent on the magnitude of thevoltage pulse and wherein both focal conic and twisted planar texturesare simultaneously stable in the absence of a field.
 5. The methodaccording to claim 4 comprising selectively switching said material to alight reflecting twisted planar structure following the sudden removalof a voltage pulse effective to homeotropically align the liquidcrystal, and a light scattering focal conic texture following removal ofa voltage pulse below that which will homeotropically align the liquidcrystal.
 6. The method according to claim 4 comprising applying squareA.C. voltage pulses.